Re: VCPU hotplug on KVM/ARM

[bthakur@codeaurora.org]

Hi Christoffer,

Thanks for your reply.

On 2018-02-27 16:17, Christoffer Dall wrote:
> Hi Bhupinder,
> 
> On Tue, Feb 27, 2018 at 03:01:17PM +0530, bthakur@codeaurora.org wrote:
>> I hope it is the right forum to post my query.
>> 
>> 
>> 
>> I am currently looking at the possibility of adding a new VCPU to a 
>> running
>> guest VM in KVM/ARM. I see that currently, it is not allowed to add a 
>> new
>> VCPU to a guest VM, if it is already initialized. The first check in
>> kvm_arch_vcpu_create() returns failure if it is already initialized.
>> 
> 
> This would require a major rework of a lot of logic surrounding the GIC
> and other parts of KVM initialization.
> 
>> 
>> 
>> There was some work done in QEMU to add support for VCPU hotplug:
>> https://lists.gnu.org/archive/html/qemu-arm/2017-05/msg00404.html
>> 
>> 
>> 
>> But I am looking at the KVM side for enabling adding a new VCPU. If 
>> you can
>> point me to any relevant work/resources, which I can refer to then it 
>> will
>> help me.
>> 
> 
> I don't have any specific pointers, but I was always told that the way
> we were going to do CPU hotplug would be to instantiate a large number
> of VCPUs, and hotplug would be equivalent to turning on a VCPU which 
> was
> previously powered off.
> 
> Is this not still a feasible solution?
It should be a feasible solution provided the guest VM is not able to 
control the onlining/offlining of VCPUs. It should be controlled by the 
Host.

> 
> How does VCPU hotplug work on x86?
On x86, you can add a vcpu through libvirt setvcpu command and it shows 
up in the guest VM as a new CPU if you do lscpu.

> 
> Thanks,
> -Christoffer

Regards,
Bhupinder